In cellular mobile radio systems, communication between a fixed radio network and mobile radios takes place on radio channels (pairs of transmitting and receiving frequencies). Radio channels used for transmission of e.g. user speech or data are called traffic channels. In radio systems according to the standard MPT 1343 issued by the British Department of Trade and Industry (DTI) or in other corresponding radio systems a radio channel reserved for signalling between a fixed network and a mobile radio is called a control channel. A control channel also identifies the system and the site of the base station thus functioning as a kind of beacon for mobile radios searching for a suitable control channel.
In two hunt sequences for a control channel defined in the standard MPT 1343, i.e. in an NH sequence (normal hunt sequence) and a CH sequence (comprehensive hunt sequence), a mobile radio scans a number of channels on searching for a suitable control channel (base station) as the connection on the current channel is weakening or lost or a mobile radio is turned on. In the NH sequence a mobile radio scans all the radio channels assigned as control channels. In the CH sequence a mobile radio scans radio channels that are usually traffic channels but may also be used as control channels.
The NH channels are permanently programmed in a mobile radio. It is possible for a radio system to add a new radio channel to the list of NH channels or to withdraw a radio channel therefrom by means of particular signalling messages.
A large trunking system may include even over a thousand radio channels, a principally arbitrary group of which may be radio channels of the CH sequence. However, a list of NH channels to be stored in a mobile radio may usually not include more than 32 channels. It as then important in the system to keep the total number of control channels as close to the number of NH channels as possible to minimize the time the mobile radios require for hunting control channels.
A known solution is to assign an NH channel as a control channel to each base station. However, this is not always possible and, in addition, in the course of time the system will have to change control channels of base stations e.g. due to faults in radio units. A currently free radio channel capable of functioning as a control channel is then assigned as a new control channel. The channel selected is often other than an NH channel. In the course of time, many of the control channels of the system may be CH channels, whereby the hunt sequences for control channels become longer.
If an NH control channel is withdrawn from a base station of the system and said channel is not currently used as a control channel in any other part of the system, the system will have to send information on the withdrawal on all control channels. If, on the other hand, a radio channel that is not used anywhere else in the system is assigned to a base station as a new control channel, this should be announced to all the mobile radios. If there are numerous changes of channel and channels are allocated as control channels at random, the above messages load the control channels heavily.